Consider replacing the "Applications" menu with a button to the "Activities" overview

See the discussion on https://lists.autistici.org/message/20190831.084510.f7549ca4.en.html.

I should be relatively easy to replace the “Activities” button with a “Applications ⏷” and thus have the exact same discoverability for it.

We should still be careful about the usability of the Activities overview itself and make sure that it’s actually better than the current state of things before doing the change.

It might be possible to always display a dock, like Ubuntu does.